Semiconductor device

ABSTRACT

According to the present invention, there is provided a semiconductor device including a MOSFET, comprising: a second-conductivity-type semiconductor layer selectively formed in one surface portion of a first first-conductivity-type semiconductor layer; a second first-conductivity-type semiconductor layer selectively formed in a surface portion of said second-conductivity-type semiconductor layer; a first main electrode electrically connected to said second first-conductivity-type semiconductor layer and second-conductivity-type semiconductor layer; a second main electrode electrically connected to the other surface of said first first-conductivity-type semiconductor layer; and a control electrode formed on the surfaces of said second first-conductivity-type semiconductor layer, second-conductivity-type semiconductor layer, and first first-conductivity-type semiconductor layer via an insulating film, and a junction between said second main electrode and first first-conductivity-type semiconductor layer is a Schottky contact.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims benefit of priority under 35USC §119 from the Japanese Patent Application No. 2004-356980, filed onDec. 9, 2004, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device.

MOSFETs are widely used in power supply circuits.

FIG. 17 shows the circuit configuration of a unidirectional insulatedDC-DC converter using conventional MOSFETs.

A capacitance element C100, a series circuit of a switching element M101and diode D101, and a series circuit of a diode D102 and switchingelement M102 are connected in parallel between input terminals IN101 andIN102. The primary side of a transformer T101 is connected to theconnecting point of the switching element M101 and diode D101 and theconnecting point of the diode D102 and switching element M102. A diodeD111 and inductance element L111 are connected in series between onesecondary side of the transformer T101 and an output terminal OUT100.The other secondary side of the transformer T101 is connected to anoutput terminal OUT101. A diode D112 is connected to the output terminalOUT101 and the connecting point of the diode D111 and inductance elementL111.

This circuit has a bridge arrangement, and an electric current I100flows in a direction indicated by the arrow when the switching elementsM101 and M102 are turned on. Accordingly, when the two switchingelements M101 and M102 are turned on, the electric current I100gradually increases. When the electric current I100 has reached amaximum value, the two switching elements M101 and M102 are turned off.Since this turns off the electric current I100, the electric currentI100 gradually reduces. Consequently, the electric current I100 havingthe shape of a triangular wave flows to the primary side of thetransformer T100.

As described above, the conventional circuit configuration is not aninverter bridge circuit capable of bidirectionally transmitting energy,i.e., the energy flows only from the primary side to the secondary sideof the transformer T100.

The following references disclose the conventional DC-DC converters.

Reference 1: U.S. Pat. No. 5,915,179

Reference 2: U.S. Pat. No. 5,693,569

Reference 3: U.S. Pat. No. 5,614,749

It is conventionally impossible to form a bidirectional DC-DC converterby using MOSFETs for the following reasons.

When a MOSFET is turned on, an electric current flows from the drain tothe source. An inverter operation has a mode in which energy from a loadflows to a power supply through a diode. In this mode, even when a diodefor preventing a reverse electric current is connected in parallelbetween the drain and source of the MOSFET, a body diode formed by aP-type base, N-type drift, and N-type substrate operate in the MOSFET.When the threshold voltage of a body diode of the MOSFET is, e.g., about0.8 V, this diode is forward-biased and turned on if the drain potentialbecomes lower by about 0.8 V or more than the source potential.

Since the body diode is a bipolar operating element, it cannot operateat high speed. This makes it impossible to raise the speed of theswitching operation of the inverter.

Even when a high-speed element such as a Schottky barrier diode usingsilicon carbide (to be referred to as SiC hereinafter) is used as adiode, the silicon body diode parasitic on the MOSFET has a large numberof stored carriers, and hence cannot operate at high speed.

When an IGBT (Insulated Gate Bipolar Transistor) is used as a switchingelement, no reverse electric current flows through this IGBT, so no bodydiode operates unlike in the MOSFET.

Since, however, the IGBT itself is a bipolar element, its operatingspeed is lower than that of the MOSFET formed on a silicon substrate.This also makes it impossible to provide a bidirectional DC-DC convertercapable of switching at high speed.

Especially when a unipolar high-speed diode such as a Schottky barrierdiode using SiC appears, high speed operation of a switching element isrequired. However, it cannot be achieved because of a low speed of abody diode of the MOSFET, even if a Schottky barrier diode using siliconcarbide operates at high speed.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided asemiconductor device including a MOSFET, said MOSFET comprising:

a second-conductivity-type semiconductor layer selectively formed in onesurface portion of a first first-conductivity-type semiconductor layer;

a second first-conductivity-type semiconductor layer selectively formedin a surface portion of said second-conductivity-type semiconductorlayer;

a first main electrode electrically connected to said secondfirst-conductivity-type semiconductor layer and second-conductivity-typesemiconductor layer;

a second main electrode electrically connected to the other surface ofsaid first first-conductivity-type semiconductor layer; and

a control electrode formed on the surfaces of said secondfirst-conductivity-type semiconductor layer, second-conductivity-typesemiconductor layer, and first first-conductivity-type semiconductorlayer via an insulating film, and

a junction between said second main electrode and firstfirst-conductivity-type semiconductor layer is a Schottky contact.

According to one aspect of the present invention, there is provided asemiconductor device including a MOSFET, said MOSFET comprising:

a first second-conductivity-type semiconductor layer selectively formedin one surface portion of a first first-conductivity-type semiconductorlayer formed in one surface portion of a first-conductivity-typesemiconductor substrate;

a second first-conductivity-type semiconductor layer formed selectivelyin a surface portion of said second-conductivity-type semiconductorlayer;

a first main electrode electrically connected to said secondfirst-conductivity-type semiconductor layer and firstsecond-conductivity-type semiconductor layer;

a third first-conductivity-type semiconductor layer or secondsecond-conductivity-type semiconductor layer formed in the other surfaceportion of said first-conductivity-type semiconductor substrate, andhaving an impurity concentration lower than that of saidfirst-conductivity-type semiconductor substrate;

a second main electrode electrically connected to said thirdfirst-conductivity-type semiconductor layer or secondsecond-conductivity-type semiconductor layer; and

a control electrode formed on the surfaces of said secondfirst-conductivity-type semiconductor layer, firstsecond-conductivity-type semiconductor layer, and firstfirst-conductivity-type semiconductor layer via an insulating film, and

a junction between said second main electrode and firstfirst-conductivity-type semiconductor layer is a Schottky contact.

According to one aspect of the present invention, there is provided asemiconductor device, comprising:

a MOSFET having,

a second-conductivity-type semiconductor layer selectively formed in onesurface portion of a first first-conductivity-type semiconductor layer,

a second first-conductivity-type semiconductor layer selectively formedin a surface portion of said second-conductivity-type semiconductorlayer,

a first main electrode electrically connected to said secondfirst-conductivity-type semiconductor layer and second-conductivity-typesemiconductor layer, and connected to a source terminal,

a second main electrode electrically connected to the other surface ofsaid first first-conductivity-type semiconductor layer, and

a control electrode formed on the surfaces of said secondfirst-conductivity-type semiconductor layer, second-conductivity-typesemiconductor layer, and first first-conductivity-type semiconductorlayer via an insulating film, and

a diode having a cathode connected to said second main electrode, and ananode connected to a drain terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the arrangement of a DC-DC converteraccording to the first embodiment of the present invention;

FIG. 2 is a circuit diagram showing the arrangement of MOSFETs usable asswitching elements and diodes in the DC-DC converter shown in FIG. 1;

FIG. 3 is a circuit diagram showing the arrangement of MOSFETs usable asswitching elements and diodes in the DC-DC converter shown in FIG. 1;

FIG. 4 is a longitudinal sectional view showing the operation state ofthe MOSFET when it is ON;

FIG. 5 is a view showing the potential in the operation state shown inFIG. 4;

FIG. 6 is a longitudinal sectional view showing the operation state ofthe MOSFET when it is OFF;

FIG. 7 is a view showing the potential in the operation state shown inFIG. 6;

FIG. 8 is a longitudinal sectional view showing the operation state ofthe MOSFET when a reverse bias is applied to it;

FIG. 9 is a view showing the potential in the operation state shown inFIG. 8;

FIG. 10 is a longitudinal sectional view showing an example of thestructure of a MOSFET usable as a switching element in a DC-DC converteraccording to the second embodiment of the present invention;

FIG. 11 is a longitudinal sectional view showing an example of thestructure of a MOSFET usable as a switching element in a DC-DC converteraccording to the third embodiment of the present invention;

FIG. 12 is a longitudinal sectional view showing an example of thestructure of a MOSFET usable as a switching element in a DC-DC converteraccording to the fourth embodiment of the present invention;

FIG. 13 is a longitudinal sectional view showing an example of thepackage structure of a MOSFET usable as a switching element and a diodein the DC-DC converter according to any one of the first to fourthembodiments;

FIG. 14 is a longitudinal sectional view showing another example of thepackage structure of a MOSFET usable as a switching element and a diodein the DC-DC converter according to any one of the first to fourthembodiments;

FIG. 15 is a longitudinal sectional view showing another example of thepackage structure of a MOSFET usable as a switching element and a diodein the DC-DC converter according to any one of the first to fourthembodiments;

FIG. 16 is a longitudinal sectional view showing another example of thepackage structure of a MOSFET usable as a switching element and a diodein the DC-DC converter according to any one of the first to fourthembodiments;

FIG. 17 is a circuit diagram showing the arrangement of a conventionalDC-DC converter.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings.

First Embodiment

FIG. 1 shows the arrangement of the first embodiment of the presentinvention, and a MOSFET used in each of the switching elements M1 to M4and M11 to M14 has an arrangement shown FIG. 2 to prevent a reverseelectric current produced by a body diode of the MOSFET.

A P-type base layer (P-type well) PW1 is selectively formed in thesurface portion of an N-type drift layer ND1. In the surface portion ofthe P-type base layer PW1, N-type source layers N1 and N2 are formedwith a predetermined spacing between them.

The construction of the MOSFET is symmetrical with respect to the centerof the cell, and a plurality of cells are formed regularly in thelateral direction (in the horizontal direction of FIG. 2), or in thelateral and width directions (in the vertical directions against FIG.2).

On one surface of the N-type drift layer ND1, a source electrode (firstmain electrode) S is formed to be electrically connected to the N-typesource layer N1, P-type base layer PW1, and N-type source layer N2. Asource voltage is applied to the source electrode S.

A drain electrode (second main electrode) D is formed to be electricallyconnected to the other surface of the N-type drift layer ND1. A drainvoltage is applied to the drain electrode D.

On one surface of the N-type drift layer ND1, a control electrode G1 isformed via an insulating film (not shown) so as to extend over theN-type source layer N1, P-type base layer PW1, and N-type drift layerND1. Likewise, a control electrode G2 is formed via an insulating film(not shown) so as to extend over the N-type source layer N2, P-type baselayer PW1, and N-type drift layer ND1. A common control voltage isapplied to the control electrodes G1 and G2.

Between the drain electrode D and a drain terminal D of this MOSFET, adiode D21 having an anode connected to the drain terminal D and acathode connected to the drain electrode D is formed. In addition, as isalso shown in FIG. 1, the diode D1 is connected by inverse-parallelconnection between the drain terminal D and the source electrode S ofthe MOSFET. That is, the anode is connected to the source electrode S,and the cathode is connected to the drain terminal D.

The diode D1 is a Schottky barrier diode made of, e.g., SiC which uses asemiconductor material having a bandgap of, e.g., 2 V or more. The diodeD21 is a Schottky barrier diode, and a barrier height of the Schottkycontact between the anode metal and the semiconductor layer is allowedto be lower than that of the general Schottky barrier diode, can beequal to or lower than 0.9 eV, and can be substantially 0.6 eV.

In a normal operation, an electric current flows forward from the drainterminal D to the drain electrode D and source electrode S via the diodeD21.

In a reverse-biased state in which the source voltage is higher by athreshold voltage (e.g., about 0.8 V) or more than the drain voltage, anelectric current flows from the source electrode S to the drain terminalD via the diode D1.

In addition, the diode D21 blocks an electric current which flowsthrough a body diode formed by the N-type drift layer ND1 and P-typebase layer PW1 of the MOSFET. As a consequence, no electric currentflows to the body diode parasitic on the MOSFET, so a reverse electriccurrent can be supplied to the Schottky barrier diode D1 capable ofhigh-speed operation. This realizes a high-speed operation in the DC-DCconverter using MOSFETs.

In this case, a relationship indicated byForward voltage of diode D1<reverse breakdown voltage of diode D21   (1)is preferable.

When an electric current flows forward in the diode D1, an avalancheelectric current flows through the diode D21 if the reverse breakdownvoltage of the diode D21 is lower than the forward voltage of the diodeD1. As a consequence, an electric current flows from the source to thedrain of the MOSFET, i.e., an electric current flows through the bodydiode of the MOSFET. To avoid this phenomenon, inequality (1) describedabove is preferable.Reverse breakdown voltage of diode D21<forward blocking voltage ofMOSFET   (2)

When the MOSFET is OFF and a voltage is applied from the drain to thesource, the forward breakdown voltage of the MOSFET must be higher thanthe reverse breakdown voltage of the diode D21.Forward blocking voltage of MOSFET<reverse breakdown voltage of diode D1  (3)

When the MOSFET is OFF and a voltage is applied from the drain to thesource, an avalanche electric current flows through the MOSFET if thereverse breakdown voltage of the diode D1 is higher than the forwardbreakdown voltage of the MOSFET. That is, a sustaining state is allowedto occur in the MOSFET. This is so because the MOSFET has a larger chiparea and a lower thermal resistance than those of the diode D1, so thebreakdown voltage increases if the heat generated when the avalancheelectric current flows is given to the MOSFET. Accordingly, inequality(3) described above desirably holds although it is not essential.

The MOSFET used in the first embodiment may also have an arrangementshown in FIG. 3.

In this MOSFET, a Schottky contact SH is formed in the junction portionbetween the N-type drift layer ND1 and drain electrode D. Since theSchottky contact SH is formed, a diode in the same direction as thediode D21 shown in FIG. 2 is formed in this junction portion, so abreakdown voltage can be obtained when a reverse bias is applied.

The rest of the arrangement is the same as that shown in FIG. 2 exceptthat the diode D21 which is no longer necessary because the reversebreakdown voltage is obtained is omitted, so an explanation thereof willbe omitted.

The reverse breakdown voltage obtained by the Schottky contact need onlybe higher than the voltage drop when the Schottky barrier diode D1connected by inverse-parallel connection to the MOSFET is ON, and can belower than the forward blocking voltage of the MOSFET. For example, thisreverse breakdown voltage need only be about 3 V or more.

The operations of the MOSFET shown in FIG. 3 will be explained below.

(1) ON State

FIG. 4 is a longitudinal sectional view showing the operation of theMOSFET when it is turned on in the forward direction. Also, referring toFIG. 5, the abscissa indicates the direction of depth from the drain tothe source, and the ordinate indicates the potential (eV) of the driftlayer and Schottky contact SH.

An electric current flows from the drain to the source as indicated bythe arrows shown in FIG. 4. Accordingly, electrons flow from the sourceto the drain, and the voltage drop in the ON state appears in theSchottky contact between the anode metal and the semiconductor layer SH.

A barrier height of the Schottky contact between the anode metal and thesemiconductor layer SH is allowed to be lower than that of the generalSchottky barrier diode, can be equal to or lower than 0.9 eV, and can besubstantially 0.6 eV.

(2) Forward OFF State

FIG. 6 is a longitudinal sectional view showing the operation when theMOSFET is forward blocking condition. FIG. 7 shows the potential in thiscase.

As shown in FIG. 6, a portion as a main junction between the P-type baselayer PW1 and N-type drift layer ND1 is depleted. This is the sameoperation state as that of a normal MOSFET, so this state is an OFFstate in which no electric current flows.

(3) Reverse Blocking State

FIG. 8 is a longitudinal sectional view showing the operation when theMOSFET is in a reverse blocking state. FIG. 9 shows the potential inthis case.

A region near the Schottky barrier layer SH is depleted, and this blocksan electric current which flows into the drain from the source.

The reverse breakdown voltage of the diode D1 connected byinverse-parallel connection is desirably higher than the forwardbreakdown voltage of the MOSFET.

This is so because, as described above, if avalanche occurs in asustaining mode or the like, heat concentrates to the diode because itschip size is smaller than that of the MOSFET, but the degree ofconcentration of heat in the MOSFET is smaller than that in the diodebecause the chip size of the MOSFET is larger than that of the diode.

In the conventional MOSFET, as described earlier, when a reverse bias isapplied to the drain-to-source path, the body diode formed by the P-typebase layer PW1 and N-type drift layer ND1 is forward-biased to perform abipolar operation, so the operating speed lowers. In this embodiment, anoperation like this is inhibited, and the high-speed Schottky barrierdiode D1 is connected by inverse-parallel connection to supply a reverseelectric current to this portion, thereby achieving a high-speedoperation.

Incidentally, a barrier height of the Schottky contact between the anodemetal and the semiconductor layer SH is allowed to be lower than that ofthe general Schottky barrier diode, can be equal to or lower than 0.9eV, and can be substantially 0.6 eV.

The portion of the Schottky contact between the anode metal and thesemiconductor layer SH has a function for turning a current, which isabout to flow into a body diode of the MOSFET, toward the Schottkybarrier diode D1. Therefore, allowable leak current is not so low,compared with the conventional Schottky barrier diode. Not less than onehundred is sufficient for a ratio of on and off current (a ratio ofconductive and leak current).

A voltage drop in the Schottky contact between the anode metal and thesemiconductor layer SH can be reduced and loss at on state can belowered by lowering the barrier height.

The DC-DC converter according to the first embodiment is widelyapplicable to, e.g., a small-sized loop controller, bidirectionaloffline power supply, adaptor, and insulated inverter.

FIG. 1 shows the arrangement of the bidirectional DC-DC converter usingMOSFETs shown in FIG. 2, and this converter can bidirectionally supplyenergy between the primary side and secondary side of a transformer T1.

The portion of the Schottky contact between the anode metal and thesemiconductor layer SH has a function for turning a current, which isabout to flow into a body diode of the MOSFET, toward the Schottkycontact between the anode metal and the semiconductor layer.

A capacitance element C1, series-connected switching elements M1 and M2,and series-connected switching elements M3 and M4 are connected inparallel between input terminals IN1 and IN2.

The primary side of the transformer T1 is connected to the connectingpoint of the switching elements M1 and M2, and to the connecting pointof the switching elements M3 and M4.

A capacitance element C2, series-connected switching elements M11 andM12, and series-connected switching elements M13 and M14 are connectedin parallel between output terminals OUT1 and OUT2.

The secondary side of the transformer T1 is connected to the connectingpoint of the switching elements M11 and M12, and to the connecting pointof the switching elements M13 and M14.

In addition, diodes D1 to D4 are connected in parallel between thedrains and sources of the switching elements M1 to M4, and diodes D11 toD14 are connected in parallel between the drains and sources of theswitching elements M11 to M14. The diode D1, for example, has an anodeconnected to the connecting point of the switching elements M1 and M2,and a cathode connected to the input terminal IN1.

This circuit has a bridge configuration; an electric current flows inone way when the switching elements M1 and M4 are turned on and theswitching elements M2 and M3 are turned off, and flows in the other waywhen the switching elements M2 and M3 are turned on and the switchingelements M1 and M4 are turned off.

A switching control circuit SWC controls the ON/OFF operations of theswitching elements M1 to M4 and M11 to M14. The switching controlcircuit SWC receives a control signal from a central control unit (notshown) or the like, and generates and supplies switching control signalsSSW1 to SSW4 and SSW11 to SSW14 to the switching elements M1 to M4 andM11 to M14.

The MOSFETs used for the switching elements M1 to M4 and M11 to M14 hasa configuration shown in FIG. 2 to prevent a reverse electric currentcaused by the body diode.

Second Embodiment

A semiconductor device according to the second embodiment of the presentinvention will be described below. The circuit configuration of theentire device is the same as that shown in FIG. 1, and an explanationthereof will be omitted.

The second embodiment differs from the first embodiment in thearrangement of a MOSFET, and FIG. 10 shows the longitudinal sectionalstructure of the MOSFET.

In the first embodiment as shown in FIGS. 2 and 3, the drain electrode Dis formed on the surface of the N-type drift layer ND1. In the secondembodiment, however, an N-type drift layer ND1 is formed on one surfaceof an N⁺-type semiconductor substrate NS1, an N⁻-type lightly dopedlayer (e.g., the impurity concentration is 1×10¹⁷/cm³ or less) NL isformed on the other surface, and a drain electrode D is formed on thesurface of the N⁻-type lightly doped layer NL. A Schottky contact SH isformed between the N⁻-type lightly doped layer NL and drain electrode D.To form the Schottky contact SH, the N⁻-type lightly doped layer NL isdesirably formed as described above.

When this MOSFET is to be used as a switching element, the anode andcathode of a diode D1 are connected to a source electrode S and thedrain electrode D, respectively, as shown in FIGS. 1 and 3, so electriccurrent is commutated to the anti-parallel diode D1 without through thebody diode when a reverse bias is applied.

The same effects as in the first embodiment can be obtained even whenthe N-type drift layer ND1 is formed on the surface of the N⁺-typesemiconductor substrate NS1 as described above.

Further, in the first embodiment, a thin wafer made of only an N⁻-typelayer is required. Therefore, it is difficult to be manufactured becausethere is a problem of a warp in a wafer, or the like.

By contrast, according to the present second embodiment, it is possibleto manufacture the device using a thick wafer by increasing thethickness of the N⁺-type layer. As a result, the device can bemanufactured easily.

Third Embodiment

A semiconductor device according to the third embodiment of the presentinvention will be described below. The circuit configuration of theentire device is the same as that shown in FIG. 1, and an explanationthereof will be omitted.

The third embodiment uses a MOSFET having a longitudinal sectionalstructure shown in FIG. 11.

In the third embodiment, an N-type drift layer ND1 is formed on onesurface of an N⁺-type semiconductor substrate NS1, a P-type impuritydiffusion layer PL is formed on the other surface, and a drain electrodeD is formed on the surface of the P-type impurity diffusion layer PL.

The P-type impurity layer PL and the N⁺-type semiconductor substrate NS1form a P-N junction, so that the junction forms a diode in the samedirection as D21 shown in FIG. 2.

Also, when the P-type impurity diffusion layer PL is formed, holes flowinto the N⁺-type semiconductor substrate NS1. However, in the N⁺-typesemiconductor substrate NS1, these holes recombine with electrons anddisappear because the length of the N⁺-type semiconductor substrate NS1is much longer than the hole diffusion length in the N⁺-typesemiconductor substrate NS1. For example, the length of NS1 is 80 μm, orlarger than the thickness of the drift layer.

When this MOSFET is to be used as a switching element, the anode andcathode of a diode D1 are connected to a source electrode S and thedrain electrode D, respectively, as shown in FIGS. 1 and 3, so thatelectric current is commutated to the anti-parallet diode D1 withoutthrough the body diode when a reverse bias is applied.

The same effects as in the first and second embodiments can be obtainedeven when the N-type drift layer ND1 is formed on one surface of theN⁺-type semiconductor substrate NS1 and the P-type impurity layer PL isformed on the other surface as described above.

Incidentally, an epitaxial layer can be used instead of the P-typeimpurity layer PL. An impurity concentration can be reduced, so thatinjection of holes is suppressed. As a result, high speed of operationcan be achieved.

Furthermore, a metal junction of a reverse side (drain) of the wafer canbe ohmic contact, thus the conventional manufacturing process can beused. In addition, the property of the device is stable because P-Njunction is used.

Fourth Embodiment

A semiconductor device according to the fourth embodiment of the presentinvention will be described below. The circuit configuration of theentire device is the same as that shown in FIG. 1, and an explanationthereof will be omitted.

The fourth embodiment uses a MOSFET having a longitudinal sectionalstructure shown in FIG. 12. P⁺-type impurity diffusion layers P11 andP12 are formed in the surface portion, in which a drain electrode D isformed, of an N-type drift layer ND1. A Schottky contact SH is formedbetween the N-type drift layer ND1 and drain electrode D.

The fourth embodiment differs from the third embodiment in that theN-type drift layer ND1 is not formed on an N⁺-type semiconductorsubstrate NS1 having a sufficient length. Therefore, to prevent manyholes from flowing into the N-type drift layer ND1 from the P⁺-typeimpurity diffusion layers P11 and P12, these diffusion layers are notformed as continuous layers but divided into a plurality of portions.

A diode is formed in a direction in which it blocks an electric currentwhich flows into the body diode between the P⁺-type impurity diffusionlayers P11 and P12 and the N-type drift layer ND1 in the fourthembodiment as well.

In the fourth embodiment, as in the first to third embodiments describedabove, no electric current flows into the body diode even when a reversebias is applied, so a high-speed operation can be realized. The Schottkymetal can be platinum, gold, titanium, or tungsten, and may also bealuminum if the concentration in the interface is 1×10¹⁷/cm³ or less.

A package structure when the semiconductor device according to any ofthe first to fourth embodiments is to be packaged will be explainedbelow.

FIG. 13 shows an example of the longitudinal sectional structure of thepackage. A chip 12 of a MOSFET and a chip 13 of at least one diode aremounted on a bed 11 of a lead frame. The chips 12 and 13 and a lead 10are connected by bonding wires, and the entire package is encapsulatedwith molding resin 14.

Examples of the planar structure of this package are shown in FIGS. 14,15, and 16.

The planar structure shown in FIG. 14 is equivalent to the structureusing the two diodes D1 and D21 as shown in FIG. 2 of the firstembodiment.

A chip 31 of the MOSFET and a chip 32 of the diode D21 are mounted on abed 21, and a chip 33 of the diode D1 is mounted on a lead 22 connectedto a drain electrode D. A lead 23 is connected to a control electrode G,and a lead 24 is connected to a source electrode S. The source electrodeS of the chip 31 of the MOSFET is connected to the lead 24 by a bondingwire. The control electrode G is connected to the lead 23 by a bondingwire. The anode of the diode D21 which is mounted such that its cathodeis in contact with the bed 21 is connected to the lead 22 by a bondingwire. The anode of a diode D33 which is mounted such that its cathode isin contact with the lead 22 is connected to the lead 24 by a bondingwire. The entire package is encapsulated with molding resin (not shown).

The planar structure shown in FIG. 15 is also equivalent to thestructure using the two diodes D1 and D21 shown in FIG. 2.

A chip 51 of the MOSFET and a chip 52 of the diode D21 are mounted on abed 41, and a chip 53 of the diode D21 is mounted on a lead 42 connectedto a drain electrode D. A lead 43 is connected to a control electrode G,and a lead 44 is connected to a source electrode S. The individualelectrodes are connected to the leads 42 to 44 by bonding wires. Theentire package is encapsulated with molding resin (not shown).

Compared to the package structure shown in FIG. 14, the packagestructure shown in FIG. 15 simplifies the shapes and arrangements of thebed 41 and leads 42 to 44, and facilitates the manufacture of the metalmold of the lead frame.

The planar structure shown in FIG. 16 is equivalent to the structureusing the diode D1 as shown in FIG. 3 of the first embodiment.

A chip 71 of the MOSFET and a chip 72 of the diode D1 are mounted on abed 61 integrated with a lead connected to a drain electrode D. A lead62 is connected to a control electrode G, and a lead 63 is connected toa source electrode S. The individual electrodes are connected to theleads 62 and 63 by bonding wires. The entire package is encapsulatedwith molding resin (not shown).

This package structure makes the number of parts smaller and themanufacture of the lead frame metal mold easier than those of thepackage structure shown in FIG. 15.

In this structure, therefore, the parasitic capacitance between the chip71 of the MOSFET and the chip 72 of the diode can be greatly reduced bymounting them on the bed 61 of the same lead frame and encapsulatingthem with the same molding resin.

In the semiconductor devices according to the above embodiments, in eachof the MOSFETs used as the first to eighth switching elements, aSchottky contact is formed between the first-conductivity-typesemiconductor layer and second main electrode. When a reverse bias isapplied, therefore, an electric current flows not to the body diode butto the first to eighth diodes connected by inverse-parallel connection.This realizes a high-speed operation.

Each of the above embodiments is merely an example, and hence does notlimit the present invention. Accordingly, these embodiments can bevariously modified within the technical scope of the invention. Forexample, the upper construction of the MOSFET may have a trench gate.

1. A semiconductor device including a MOSFET, said MOSFET comprising: asecond-conductivity-type semiconductor layer selectively formed in onesurface portion of a first first-conductivity-type semiconductor layer;a second first-conductivity-type semiconductor layer selectively formedin a surface portion of said second-conductivity-type semiconductorlayer; a first main electrode electrically connected to said secondfirst-conductivity-type semiconductor layer and second-conductivity-typesemiconductor layer; a second main electrode electrically connected tothe other surface of said first first-conductivity-type semiconductorlayer; and a control electrode formed on the surfaces of said secondfirst-conductivity-type semiconductor layer, second-conductivity-typesemiconductor layer, and first first-conductivity-type semiconductorlayer via an insulating film, and a junction between said second mainelectrode and first first-conductivity-type semiconductor layer is aSchottky contact.
 2. A semiconductor device including a MOSFET, saidMOSFET comprising: a first second-conductivity-type semiconductor layerselectively formed in one surface portion of a firstfirst-conductivity-type semiconductor layer formed in one surfaceportion of a first-conductivity-type semiconductor substrate; a secondfirst-conductivity-type semiconductor layer formed selectively in asurface portion of said second-conductivity-type semiconductor layer; afirst main electrode electrically connected to said secondfirst-conductivity-type semiconductor layer and firstsecond-conductivity-type semiconductor layer; a thirdfirst-conductivity-type semiconductor layer or secondsecond-conductivity-type semiconductor layer formed in the other surfaceportion of said first-conductivity-type semiconductor substrate, andhaving an impurity concentration lower than that of saidfirst-conductivity-type semiconductor substrate; a second main electrodeelectrically connected to said third first-conductivity-typesemiconductor layer or second second-conductivity-type semiconductorlayer; and a control electrode formed on the surfaces of said secondfirst-conductivity-type semiconductor layer, firstsecond-conductivity-type semiconductor layer, and firstfirst-conductivity-type semiconductor layer via an insulating film, anda junction between said second main electrode and said thirdfirst-conductivity-type semiconductor layer or secondsecond-conductivity-type semiconductor layer is a Schottky junctioncontact.
 3. A semiconductor device including a MOSFET, said MOSFETfurther comprising: a second second-conductivity-type semiconductorlayer selectively formed in the other surface portion of said firstfirst-conductivity-type semiconductor layer; wherein a second mainelectrode is electrically connected to the other surface of said firstfirst-conductivity-type semiconductor layer and said secondsecond-conductivity-type semiconductor layer; and a control electrode isformed on the surfaces of said second first-conductivity-typesemiconductor layer, first second-conductivity-type semiconductor layer,and first first-conductivity-type semiconductor layer via an insulatingfilm.
 4. A semiconductor device comprising: a MOSFET having, asecond-conductivity-type semiconductor layer selectively formed in onesurface portion of a first first-conductivity-type semiconductor layer,a second first-conductivity-type semiconductor layer selectively formedin a surface portion of said second-conductivity-type semiconductorlayer, a first main electrode electrically connected to said secondfirst-conductivity-type semiconductor layer and second-conductivity-typesemiconductor layer, and connected to a source terminal, a second mainelectrode electrically connected to the other surface of said firstfirst-conductivity-type semiconductor layer, and a control electrodeformed on the surfaces of said second first-conductivity-typesemiconductor layer, second-conductivity-type semiconductor layer, andfirst first-conductivity-type semiconductor layer via an insulatingfilm; and a diode having a cathode connected to said second mainelectrode, and an anode connected to a drain terminal.
 5. A deviceaccording to claim 1, further comprising: a bridge circuit having afirst capacitance element connected in series between first and secondinput terminals, and first and second switching elements connected inseries between said first and second input terminals, said first andsecond switching elements respectively consisted of said MOSFET.
 6. Adevice according to claim 2, further comprising: a bridge circuit havinga first capacitance element connected in series between first and secondinput terminals, and first and second switching elements connected inseries between said first and second input terminals, said first andsecond switching elements respectively consisted of said MOSFET.
 7. Adevice according to claim 3, further comprising: a bridge circuit havinga first capacitance element connected in series between first and secondinput terminals, and first and second switching elements connected inseries between said first and second input terminals, said first andsecond switching elements respectively consisted of said MOSFET.
 8. Adevice according to claim 4, further comprising: a bridge circuit havinga first capacitance element connected in series between first and secondinput terminals, and first and second switching elements connected inseries between said first and second input terminals, said first andsecond switching elements respectively consisted of said MOSFET.
 9. Adevice according to claim 1, said device further comprising, a Schottkybarrier diode having an anode connected to said first main electrode ofsaid MOSFET, and a cathode connected to said second main electrode ofsaid MOSFET.
 10. A device according to claim 2, said device furthercomprising, a Schottky barrier diode having an anode connected to saidfirst main electrode of said MOSFET, and a cathode connected to saidsecond main electrode of said MOSFET.
 11. A device according to claim 3,said device further comprising, a Schottky barrier diode having an anodeconnected to said first main electrode of said MOSFET, and a cathodeconnected to said second main electrode of said MOSFET.
 12. A deviceaccording to claim 4, said device further comprising, a Schottky barrierdiode having an anode connected to said first main electrode of saidMOSFET, and a cathode connected to said drain terminal.
 13. A deviceaccording to claim 1, wherein said MOSFET and a diode connected inparallel to said MOSFET are mounted on the same lead frame in the samepackage.
 14. A device according to claim 2, wherein said MOSFET and adiode connected in parallel to said MOSFET are mounted on the same leadframe in the same package.
 15. A device according to claim 3, whereinsaid MOSFET and a diode connected in parallel to said MOSFET are mountedon the same lead frame in the same package.
 16. A device according toclaim 4, wherein said MOSFET and a diode connected in parallel to saidMOSFET are mounted on the same lead frame in the same package.
 17. Adevice according to claim 1, wherein a barrier height of the Schottkycontact is equal to or lower than 0.9 eV.
 18. A device according toclaim 2, wherein a barrier height of the Schottky contact is equal to orlower than 0.9 eV.
 19. A device according to claim 3, wherein a barrierheight of the Schottky contact is equal to or lower than 0.9 eV.
 20. Adevice according to claim 4, wherein a barrier height of the Schottkycontact is equal to or lower than 0.9 eV.